Liquid crystal display panels (LCDS), particularly color LCD panels, are used for flat screen televisions, projection television systems and camcorder view finders, with many more applications anticipated in the future.
The fabrication of an active matrix liquid crystal display involves preparation of a front and rear glass panel. Preparation of the front glass panel involves deposition of a color filter element onto a suitable substrate, such as glass. Color filter deposition typically involves depositing a black matrix pattern and three primary (typically either red, green and blue or yellow, magenta and cyan) color dot or color cell patterns within the spaces outlined by the black matrix. The lines which form the black matrix typically are about 15-25 microns wide and about 0.5 to 2 microns thick. The red, green, and blue color cells are typically on the order of about 70-100 microns in width by 200 to 300 microns in length. The printed color cells are typically less than about 10 microns thick, and preferably less than 5 microns thick, and must be evenly applied and accurately registered within the pattern formed by the black matrix. The front glass substrate is typically completed by depositing a planarizing layer, a transparent conducting layer, and a polyamide alignment layer over the color filter element. The transparent conducting layer is typically indium tin oxide (ITO), although other materials can also be utilized.
Preparation of the rear glass panel involves formation of thin film transistors or diodes, as well as metal interconnect lines. Each transistor acts as an on-off switch for an individual color pixel in the display panel. The third and final step is the assembly of the two panels, including injection of a liquid crystal material between the two panels to form the liquid crystal display panel.
One critical step in color filter formation is the formation of the red, green and blue color dots (also referred to as color cells) of the color filter. Such color cells preferably should be deposited so that they are as smooth and uniform in thickness as possible. Previous ink printing methods used to print color filter patterns have resulted in color patterns having insufficient smoothness. This is largely because the ink depositing methods of the prior art resulted in ink cells which were rounded or triangular in cross section. Consequently, a planarizing layer is commonly applied over the color patterns, after they have been printed onto a substrate, to alleviate imperfections in coating smoothness or thickness uniformity due to the deposition process. The transparent planarizing layer also serves to protect against ion migration to and from the ITO layer and color pattern layer. The planarizing layer should be deposited to be as smooth and flat as possible.
Previously, a new method for providing such protective planarizing layers has been proposed by Corning Incorporated. This new method has been the subject of a commonly owned patent application Ser. No. 08/197,141, filed Feb. 16, 1994, entitled "Color Filters and Method of Printing", the specification of which is hereby incorporated by reference. In that application, a transfer layer 14 is provided onto a collector 16. Afterwards, the ink color cells are deposited using ink printing methods onto the transparent transfer layer. The ink color cells are then deposited onto a glass substrate, and sandwiched between the transfer layer and the glass substrate, in a process similar to that illustrated in FIG. 3.
After a planarizing layer has been deposited, a conductive layer such as indium tin oxide (ITO) is typically deposited over the planarizing layer. If the outside top or upper edges of the planarizing layer have sharp or square (90 degrees) edges, breaks or discontinuities can occur in the conductive (ITO) layer which is deposited over the planarizing layer. Consequently, it would be desirable to provide a method and apparatus for making color filters which is capable of producing planarizing layers which do not have sharp edges and therefore do not result in breaks in the ITO layer. It would further be desirable to produce such color filters using a method which is simpler and more efficient than prior art methods.
In addition to the ability to provide tapered or radiused edges on the color filters, it would also be desirable to be able to produce other contoured shapes on the planarizing layer or other top protective layer which is deposited over the color filter. For example, it would be desirable to be able to impart a microgroove pattern on the surface of the top protective layer. Such microgroove patterns can be used to orient the liquid crystal without the need for a separate alignment layer. Such microgroove patterns are discussed, for example, in "Regularity and Narrowness of the Intervals of the Microgrooves on the Rubbed Polymer Surface for LC Alignment" by Ito et al., SID 1992 digest, pages 393-396 (1992).